1. Field of the Invention
This invention relates to a zoom lens for a camera, and in particular to a compact zoom lens of two-group construction chiefly for a 35 mm format single-lens reflex camera.
2. Related Background Art
There have heretofore been provided numerous two-group zoom lenses, i.e., a first lens group having a negative refractive power and a second lens group having a positive refractive power, and effecting a magnification change by the spacing therebetween being varied. Many of these two-group zoom lenses have a zoom ratio of twice or less, but some zoom lenses in which a wide angle, a high magnification (a high zoom ratio) and compactness are achieved have also been proposed.
For example, in Japanese Laid-Open Patent Application No. 62-177514, there is proposed a wide angle two-group zoom lens in which the first lens group having a negative refractive power is comprised of three spherical lenses and which has a relatively high zoom ratio.
Also, in Japanese Laid-Open Patent Application No. 58-121011, there is disclosed a wide angle two-group zoom lens which adopts a construction in which an aspherical lens is introduced into the first lens group having a negative refractive power and in which a high zoom ratio is achieved and compactness of the first lens group is realized.
Now, generally in a zoom lens of two-group construction, excessive off-axis light flux enters and therefore, the fluctuation of coma by zooming poses a great problem. Therefore, there has also been proposed a zoom lens in which the fluctuation of coma is suppressed by a construction having a flare stop operatively associated with the first lens group in response to zooming.
For example, in Japanese Laid-Open Patent Application No. 54-23555, a flare stop is provided between a first lens group having a negative refractive power and a second lens group having a positive refractive power, whereby the lower coma flare of the intermediate portion of the picture plane is effectively intercepted without causing deficiency of the quantity of marginal light and not only the fluctuation of lower coma by zooming, but also the fluctuation of coma by the angle of view is suppressed to realize an improved performance.
Also, in Japanese Laid-Open Patent Application No. 62-94812, the off-axis light ray is greatly refracted by the strong refractive power of a negative lens having its concave surface facing the image side in a second lens group having a positive refractive power and upper coma occurs and therefore, the upper coma particularly on the telephoto side is effectively intercepted by a flare stop provided at the image side of the second lens group, thereby achieving an improved performance.
Numerous efforts have been made in the known techniques as described above, but the following problems are involved therein.
In Japanese Laid-Open Patent Application No. 62-177514, distortion cannot be completely corrected by a first lens group comprised of three spherical lenses, and at the wide angle end, lower coma by color occurs greatly and further, in the medium focal length state, upper coma is over-corrected, and this is not preferable. Also, a second lens group having a positive refractive power is basically of a construction in which the refractive power is strong and therefore, correction of Petzval sum becomes difficult, and this leads to the disadvantage that the balance with astigmatism, curvature of image field and spherical aberration particularly at the telephoto end is bad.
In the zoom lens of Japanese Laid-Open Patent Application No. 58-121011, a first lens group is comprised of four lenses and therefore, the thickness of this group becomes great, and this is not preferable. Also, correction for the lower coma of the first lens group is not good and particularly, at the wide angle end, the fluctuation of lower coma by the angle of view and the lower coma by color are great and further, upper coma is over-corrected over the entire range from the wide angle end to the telephoto end.
In the zoom lens of Japanese Laid-Open Patent Application No. 54-23555, there is adopted a construction in which a wide spacing is secured between a first lens group and a second lens group to enable the flare stop as previously described to be provided, and this leads to the increased diameter of the foremost lens of the first group. Further, if the diameter of this flare stop is varied in response to zooming, it will result in the complication of the structure of the lens barrel, which in turn leads to an increased cost, and this is not preferable. Also, due to the construction in which the refractive powers in the first lens group and the second lens group are basically weak, it is difficult to obtain a compact zoom lens having a high zoom ratio.
Further, in Japanese Laid-Open Patent Application No. 62-94812, the zoom ratio is small and, as compared with the fluctuation of lower coma by zooming and the incident light flux incident on the marginal portion of the picture plane, correction of the lower coma flare, etc. of the intermediate portion of the picture plane caused by an excessive light flux entering into the intermediate portion of the picture plane is not effected, and this is not preferable.